1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to sensors placed on a mucosal surface used for sensing physiological parameters of a patient.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such characteristics of a patient. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, for example the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during different phases of the cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that transmits electromagnetic radiation, for example light, through a patient's tissue and that photoelectrically detects the absorption and scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed and scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed and scattered by the blood in an amount correlative to the amount of the blood constituent present in the tissue. The measured amount of light absorbed and scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms.
Pulse oximetry is sensitive to movement, and various types of motion may cause artifacts that may obscure the blood constituent signal. For example, motion artifacts may be caused by moving a sensor in relation to the tissue, by increasing or decreasing the physical distance between emitters and detectors in a sensor, by changing the direction of emitters or detectors with respect to tissue or each other, by changing the angles of incidence and interfaces probed by the light, by directing the optical path through different amounts or types of tissue, or by expanding, compressing or otherwise altering tissue near a sensor. In the emergency room, critical care, intensive care, and trauma center settings, where pulse oximetry is commonly used for patient monitoring, the wide variety of sources of motion artifacts includes moving of a patient or the sensor by healthcare workers, physical motion of an unanaesthetised or ambulatory patient, shivering, seizures, agitation, response to pain and loss of neural control. These motions oftentimes have similar frequency content to the pulse, and may lead to similar or even larger optical modulations than the pulse. Thus, it is desirable to reduce the movement of a pulse oximetry sensor in order to mitigate artifacts. Use of a mucoadhesive may urge the sensor into better contact with the desired site of measurement and may eliminate or reduce motion of the sensor relative to the tissue.
Alternative means of monitoring tissue constituents may also be of clinical interest. One such parameter of interest is carbon dioxide. Elevated levels of carbon dioxide in the tissue may be related to poor perfusion. Thus, assessment of carbon dioxide levels may be useful for diagnosing a variety of clinical states related to poor perfusion. One method of determining the level of blood carbon dioxide involves measuring carbon dioxide levels of respiratory gases. In relatively healthy individuals, the carbon dioxide in the bloodstream equilibrates rapidly with carbon dioxide in the lungs, the partial pressure of the carbon dioxide in the lungs approaches the amount in the blood during each breath. Accordingly, physicians often monitor respiratory gases at the end of expiration in order to estimate the carbon dioxide levels in the blood.
Respiratory gas analyzers typically function by passing electromagnetic radiation through a respiratory gas sample and measuring the absorption that is related to carbon dioxide. Often, the gas samples are collected with adapters that are fitted into patients being given respiratory assistance, for example patients under anesthesia, or patients on life support systems, to connect between the endotracheal tube (ET tube) and the ventilating tube of the breathing apparatus. These tubes convey respiratory gases to the patient and exhaled breath away from the patient. The airway adapter is in the form of a short connector of tubular shape, and is required to make a connection between the generally very different cross sections of these two tubes. Respiratory gases may also be collected through the use of cannulas, which are flexible tubes that are threaded through the mouth or nose. Respiratory gas samples collected from a cannula may be aspirated from the airway stream and exposed to a carbon dioxide sensor.
It is often inconvenient to measure carbon dioxide in respiratory gases from respiratory gas samples collected from an intubation tube or cannula. Although these methods are considered to be noninvasive, as the surface of the skin is not breached, the insertion of such devices may cause discomfort for the patient. Further, the insertion and operation of such devices also involves the assistance of skilled medical personnel.
Carbon dioxide and other physiological parameters may also be measured transcutaneously by sensors held against a patient's skin. Transcutaneously measured carbon dioxide may also be clinically useful when compared to carbon dioxide measured in respiratory gases. For example, variations in carbon dioxide measurements between these two methods may be diagnostic for certain clinical states. While transcutaneous sensors may be easier to use than respiratory gas sensors, they also have certain disadvantages. As transcutaneous sensors depend upon the perfusion of carbon dioxide through a relatively thick epidermal layer, these sensors may not be as accurate.
Direct measurement of tissue carbon dioxide, particularly in tissues sensitive to hypoperfusion, provides clinicians with important diagnostic information regarding systemic circulation and/or onset of septic shock. The oral mucosa is a tissue involved in the visceral response to systemic hypoperfusion. A sensor held in position on the oral mucosa could provide trending information about a patient's level of systemic perfusion.